Wide Band Gap semiconductor microwave materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) have higher temperature capabilities than conventional non-wide-band-gap semiconductor materials such as silicon, germanium, or low voltage gallium arsenide (GaAs). Semiconductor devices made from wide band gap materials are capable of delivering an order of magnitude greater power than non-wide-band-gap devices. This greater delivered power is accompanied by a corresponding increase in on-chip dissipated power in the form of heat.
Traditional cooling methods and materials do not have the thermal performance required to maintain chip or junction temperatures sufficiently low for adequate reliability and device performance. A SiC MESFET can dissipate (generate and transfer away to an appropriate heat sink) 5 watts per mm of gate width by comparison with the 0.5 watts per mm in a good GaAs device.
Microwave devices are ordinarily cooled by heat transfer from the active portion of the semiconductor chip, which is typically on an active surface of the semiconductor, through the semiconductor chip, and through a chip or die attachment to an underlying physical structure. The underlying structure is preferably thermally conductive, and may be associated with either fins for heat transfer to the ambient air, or with channels for the flow of liquid coolant. The chip or die attachment may be subject to stresses attributable to mismatches in the coefficient of thermal expansion (CTE) between the chip or die and the underlying structure. This CTE mismatch may be ameliorated by mounting the chip by the use of gold-tin (Au:Sn) solder to a matched-CTE shim, and mounting of the shim to the underlying high-CTE structure by means of thermally conductive adhesive. As an alternative, the chip may be directly mounted to the underlying structure by means of thermally conductive adhesive. The thermally conductive adhesives are often loaded with thermally conductive particles, but nevertheless still do not have conductivity equivalent to that of conductive metals such as copper or silver. Thermally conductive adhesives may have thermal conductivity in the range of about 1.0 to 1.5° C./watt-cm. In a typical application, as much as 30% to 50% of the temperature rise (temperature difference between the active portion of the chip and the underlying structure) may be attributable to the mounting adhesive. The mounting adhesive is a limitation in the cooling of high-power semiconductor devices, whether they be of SiC construction, GaN, or low-voltage GaAs.
Various alternative mounting/cooling arrangements have been suggested for high power microwave devices. Such alternatives include flow-through liquid cooling, micro-heat-pipes, exotic interface materials, and thermoelectric coolers. None of these solutions has met with marked success or widespread usage. Because of the continuing issues associated with high power densities and lack of suitable cooling, some SiC microwave monolithic integrated circuits (MMIC) spread the FET fingers farther apart than the feature size limitations would otherwise require, thereby sacrificing some radio-frequency (RF) performance, but also decreasing the power density in watts/mm to a level which can be handled by the heat transfer mechanisms. The increase in the size of the finger spacing in turn results in a larger FET die size, which undesirably impacts the yield of good dies per semiconductor wafer.
Improved or alternative structures and manufacturing methods are desired for electrical and thermal control in microwave semiconductor arrangements.